Two-stage flash drying system



Feb. 25, 1964 E. J. wAsP TWO-STAGE FLASH DRYING SYSTEM Filed July 15, 1960 INVENTOR- EDWARD J. WASP HIS ATTO RNEY United States Patent 3,122,425 TWQ-STAGE FLASH DRYENG SYfiiEM Edward J. Wasp, Pittsburgh, Pa, assignor to Consolidation Coal Company, Pittsburgh, Pa., a corperation of Pennsyivania Fiied .iuiy 13, 1966, fier. No. 42,5?8 7 Ciaims. (Ci. 34-10) This invention relates to an improved drying system, and more particularly to a two-stage drying system in which a pair of driers are utilized to control the final moisture content of particulate material to be dried.

Drying of particulate material can be accomplished by mixing it intimately with a hot gaseous drying medium and maintaining it in intimate contact with the gaseous medium until dried to the desired percentage of moisture content. It has been found that the more the moisture content of the dried product is reduced, the greater is the amount of dust contamination carried in the flue gases or exit gases that leave the drying area and are vented to atmosphere.

The present invention contemplates providing a drying system in which particulate material may be dried until it has a very low moisture content, and, at the same time, pollution of the atmosphere by dust entrained in the drier exit gases may be reduced to a minimum.

In the present invention, the drying of particulate material is done in two stages. Increased drying is accomplished in the first stage on a portion of the material to be dried. The heavily dust laden exit gases from the first stage are then conducted to, and utilized in, the second stage where decreased drying occurs. A second portion of the material to be dried is dried in the second stage and emerges with a greater percentage of moisture content than the first stage product. The two portions of dried material are then blended with each other to produce a quantity of the particulate material which has a moisture content greater than the product of the increased drying in the first stage and less than the product of the reduced drying from the second stage. Only the exit gases from the second stage are vented to atmosphere. Because the second stage drying is not carried to the point of an extremely dry final product, the exit gases from the second stage are not heavily dust laden.

The present invention is particularly adapted for use with flash drying systems, although not limited thereto, and will be described in detail as applied to a flash drying system. In conventional flash drying systems, it is common to temper or control the temperature of the drying gases entering the flash drier by admitting controlled quantities of ambient air into the stream of drying gases through adamper. When air is so admitted, the exit gases from the drier contain a relatively large amount of oxygen. In the present invention, the temperature of drying gases for the second stage of the flash drying system may be controlled or tempered by the exit gases from the first stage. Accordingly, the exit gases from the second stage do not have the high oxygen content that is present when ambient air is admitted to the drying gases prior to their entry into the flash drier.

Because the oxygen content of the exit gases from the second stage drier is reduced, electrostatic precipitators may be employed to completely remove the dust particles from the exit gases before they are vented to the atmosphere. Electrostatic precipitators can not be utilized in exit gases having a high oxygen content because of the possible danger of explosion.

When the exit gases from the first stage drier are utilized to temper the second stage drying gases, a lesser overall volume of gas is passed through the system to atmosphere than would be the case if two independently operating driers were utilized. Since the dust laden first 3,122,425 Patented Feb. 25, 1964 "ice stage exit gases are passed into the second stage of the drying system rather than being cleaned and vented to atmosphere, and since fresh air is not taken into the second stage to temper the drying gases as would be the case if the first stage exit gases were not utilized, the steps of cleaning and venting gas to the atmosphere from one drier and drawing clean air from the atmosphere into the second drier are omitted. Thus, a smaller volume of exit gases from the drier must be cleaned and vented to atmosphere when the two stage drying system of the present invention is utilized.

As an alternative to tempering the drying gases of the second stage with all of the exit gases from the first stage, a portion of the exit gases from the first stage may be recycled from the first stage drier back to the source of heated gases and reheated to be utilized again as drying gas. In such an instance, the dust laden exit gases from the first stage drier will not be vented to atmosphere and hence need not be cleaned. The second stage drying gases will still be tempered by relatively oxygen free first stage exit gases.

With the foregoing considerations in mind, it is a primary object of the present invention to provide an improved two-stage drying system for particulate material.

Another object of this invention is to provide a drying system that has a minimum of dust suspended in the exit gases to be vented to the atmosphere.

Another object of this invention is to provide a flash drying system with a minimum oxygen content in the exit gases to be vented to the atmosphere.

Another object of this invention is to reduce the overall volume of exit gases from a drying system which need be cleaned before being vented to atmosphere.

A further object of this invention is to provide a twostage drying system in which exit gases from the first stage are recycled and heated to serve again as drying gases for the first and second stages.

Another object of this invention is to provide a flash drying system in which electrostatic precipitators may be utilized to remove dust particles from the exit gases vented to the atmosphere.

These and other objectives achieved by this invention will become apparent as this description proceeds in conjunction with the accompanying drawing. This invention may be utilized for drying many different types of particulate material with gas as the drying medium. It may be adapted for fluidized bed drying, flash drying, or other types of drying where heated gases are utilized. However, for descriptive purposes and not by way of limitation, this system will be described as it is utilized in the flash drying of fine coal.

The single drawing accompanying this specification is a schematic representation of the novel flash drying system of this invention.

Referring to the drawing, the flash drying system of the present invention includes generally a first flash drier It a second flash drier 12, a furnace 14 to provide a source of heated gases, and a blend hopper 16 to blend the products of the first and second flash driers.

The furnace 14 has an air inlet 18 and a fuel inlet 20 which provide the material to support combustion Within the furnace. Heated gases generated within the furnace 14 are conducted to the first flash drier 16 through a heated gas conduit 22. Heated gases from furnace 14 are also conducted to the second flash drier 12 through heated gas conduit 24. An ambient air inlet conduit 26 communicates with the heated gas conduit 22. A damper 28 controls the flow of air through the ambient air inlet conduit 26. Damper 28 may be adjusted to provide controlled quantities of ambient air into conduit 22 to temper or control the temperature of the heated gases in conduit 22.

The first flash drier includes a drying unit 30 which has a heated gas inlet 32. The conduit 22 conducts heated gases from furnace 14 into heated gas inlet 32. A particulate material conveyor 34 introduces moist particulate material from'a source (not shown) into a feed bin 36. Feed bin 36 feeds the moist particulate material into a helical screw-type conveyor 38 which is driven by an electric motor 39. The helical conveyor 38 introduces the moist particulate material into the drying unit 30 through the particulate material inlet 40.

The heated gases from conduit 22 and the moist particulate material entering the inlet 40 are intimately mixed within the drying unit 38 and the particulate material is entrained in the heated gas and passes into the drying column 42 of the first flash drier 18.

The particulate material and gas pass out of the drying column 42 through the entrained material outlet 44 and into the conduit 46. The particulate material and gas, having a reduced moisture content, pass into a cyclone separator 48 through an entrained material inlet 50. In a conventional manner the particulate material and gases are separated within the cyclone separator 48. The gases pass from the separator 48 through the exit gas outlet 52. The dried particulate material passes from the separator 48 through the particulate material outlet 54. A feeder valve 55 is provided to control the rate of discharge of the dry particulate material from the cyclone separator 48.

The exit gases from the first cyclone separator 48, which is part of the first flash drier 10, pass through the gas outlet 52 into an exit gas conduit 56. Exit gas con duit 56 is divided into a recycle branch conduit 56a and a tempering branch conduit 5611. A three way, three position valve 57 permits communication of exit gas conduit 56 with either branch conduit 56a or 5612 exclusively or with branch conduits 56a and 56b simultaneously. The relative volumetric proportions of gases flowing from conduit 56 into branches 56a and 5612 may be regulated by valve 57. The recycle branch conduit 56a commu nicates with furnace 14 so that exit gases from separator 48 may be reheated in furnace 14 and conducted through heated gas conduits 22 and 24 for further drying.

The tempering branch conduit 56b communicates with the heated gas conduit 24 that conducts heated gases from furnace 14 to the second flash drier 12. A damper 58 is provided to control the quantity of tempering gas entering the heated gas conduit 24 from tempering branch conduit 56b.

The heated gases in conduit 24 enter the second flash drier 12 through the drying unit 60 which has a heated gas inlet 62. A particulate material conveyor 64 conveys moist particulate material from a source (not shown) into the feed bin 66 of the second flashrdrier 12. A helical conveyor 68 driven by electric motor 69 conveys the moist particulate material into the drying unit 60 through the particulate material inlet 70.

The particulate material and heated gases are intimately mixed within the drying unit 60 and the particulate material is entrained in the gas and passes into the drying column 72. The particulate material and gas pass out of the drying column 72 through the entrained material outlet 74 and enter the conduit 76. The stream of particulate material and gas is conducted by conduit 76 into a cyclone separator 78 included in the flash drier 12.

The stream of particulate material and gas enters the cyclone separator 78 through the entrained material inlet 80 and is separated into the dried particulate material and the exit gases. The exit gases pass from the separator 78 through the gas outlet 82 and .the dry particulate material leaves the separator 78 through the particulate material outlet 84. A feeder valve 85 is provided to control the rate of flow of the particulate material out of separator 78. t

The exit gases from the second flash drier 12 are conducted by exit gas conduit 86 into an electrostatic precipitator 88. Electrostatic precipitator 88 has a dust laden gas inlet 90 connected to conduit 86. After the dust laden gas passes through the precipitator 88 the gases leave through the clean gas outlet 92 of the precipitator 88 and are vented to atmosphere through the vent conduit 94.

A dry material conveyor 96 is provided to receive the dry material from the separator 48 of flash drier 10. A second dry material conveyor 98 is provided to receive the dry particulate material from the separator 78 of the flash drier 12. Conveyors 96 and 88 convey the respective portions of the dry particulate material to the inlet 100 of the blend hopper 16. The portions of the dry particulate material are then blended within the blend hopper 16 and pass out of the blend hopper 16 through outlet 102. A feeder valve 104 controls the rate of flow of the particulate material out of blend hopper 16. A dry material conveyor 106 may be provided to receive the product from blend hopper 16 and convey it for ultimate consumption. Although not shown, a mixing means may be provided in blend hopper 16 to intimately mix the dried particulate material fed from conveyors 96 and 98 to provide a mixture having a uniform moisture content.

The first flash drier 10 of the present invention is 0perated at increased drying to produce a particulate material product having a very low percentage of moisture content. The gases leaving the first flash drier 10 through exit conduit 56 have a relatively high amount of dust entrained therein because of the increased drying in flash drier 10. The exit gases in conduit 56 are not vented to the atmosphere, but rather are utilized either to control the temperature of the inlet heated gases to the sec ond flash drier 12 or to be recycled through furnace 14 for reheating and further drying.

The second flash drier 12 is operated at substantially reduced drying. The particulate material that is a prod uct of flash drier 12 is relatively moist as compared with the product of drier 10. However, because of the reduced drying in flash drier 12, the exit gases leaving flash drier 12 through exit conduit 86 contain very little entrained dust.

In addition, because the heated gases in conduit 24 are tempered by the exit gases from the first flash drier 10 rather than by ambient air, the oxygen content of the exit gases leaving the second flash drier 12 through conduit 86 is substantially reduced. With this substantially reduced oxygen content, the exit gases from separator 78 may be passed through the electrostatic precipitator 88 to remove the dust therefrom without danger of explo sion.

As an example of how the system of the present invention may be operated to produce dried coal having a moisture content of from 4 to 5%, exemplary temperatures and moisture contents will be provided for one possible operation. It is to be understood that the various temperatures and moisture contents may be varied in accordance with the predetermined moisture content de' sired in the end product.

In the exemplary operation, heated gases leave the furnace 14 through conduits 22 and 24 at 1500 F. The heated gas in conduit 22 is tempered by ambient air and enters the drying unit 30 at 1000 F. The first flash drier 10 is operated to produce coal having a moisture content of 3% leaving the cyclone 48. The exit gases leaving separator 48 through conduit 52 have a temperature of approximately 200 F.

The exit gases from separator 48 in branch conduit 56b are utilized to temper the heated gases in conduit 24 so that the heated gases in conduit 24 enter the drying unit 60 at a temperature of 1000 F. Any excess gases from separator 48 are recycled through branch conduit 56a to furnace 14. The second flash drier 12 is operated to produce coal having a moisture content of 8% leaving the separator 78 of flash drier 12. The exit gases leaving the separator 78 through conduit 86 have an oxygen content of less than 10% The coal leaving the cyclone separator 48 of flash drier with a moisture content of 3% and the coal leaving the cyclone separator '78 of flash drier 12 with a moisture content of 8% are blended within the blend hopper 16. The quantity of coal having the predetermined moisture content of 4 to 5% is removed from the blend hopper 16 and conveyed by dry material conveyor 106.

According to the provisions of the patent statutes, I have explained the principle, preferred construction, and mode of operation of my invention and have illustrated and described What I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

1. The method of drying a quantity of moist particulate material having a spectrum of sizes to a predetermined percentage of moisture content, comprising the steps of supplying a first stream of heated gas, controlling the temperature of said first stream by admitting ambient air into said first stream in controlled quantities, subjecting a first portion of said material to said first stream of heated gas to dry said first portion of said material to a percentage of moisture content less than said predetermined percentage of moisture content, collecting the exit gases of said first stream after said stream has dried said first portion of said material, supplying a second stream of heated gas, contro ing the temperature of said second stream by admitting at least some of said exit gases from said first stream into said second stream in controlled quantities, subjecting a second portion of said material to said second stream of heated gas to dry said second portion of said material to a percentage of moisture content greater than said predetermined percentage of moisture content, separating said second stream of heated gas from said second portion or said material, introducing said second stream of gas into an electrostatic precipitator to remove entrained fine sized dust-like material therefrom, and blending said first and second portions of said material to produce a quantity of said material having said predetermined percentage of moisture content.

2. The method of drying a quantity of moist particulate material having a spectrum of sizes to a predetermined percentage of moisture content, comprising the steps of supplying a first stream of heated gas, controlling the temperature of said first stream by admitting ambient air into said first stream in controlled quantities, subjecting a first portion of said material to said first stream of heated gas to dry said first portion of said material to a percentage of moisture content less than said predetermined percentage of moisture content, collecting the exit gases of said first stream after said stream has dried said first portion of said material, recycling at least a portion of said first stream exit gases into proximity with a source of heat to create additional drying gases, supplying a second stream of heated gas, controlling the temperature of said second stream, subjecting a second portion of said material to said second stream or" heated gas to dry said second portion of said material to a percentage of moisture content greater than said predetermined percentage of moisture content, separating said second stream of heated gas from said second portion of said material, said second stream of heated gas containing less oxygen than said first stream of heated gas, introducing said second stream of said heated gas into an electrostatic precipitator to remove entrained fine sized dust-like material therefrom, and blending said first and second portions of said material to produce a quantity of said material having said predetermined percentage of moisture content.

3. The method of drying a quantity of moist particulate material having a spectrum of sizes to a predetermined ercentage of moisture content comprising the steps of 6 supplying a first stream of heated gases, controlling the temperature of said first stream by admitting ambient air into said first stream in controlled quantities, subjecting a first portion of said material to said first stream of heated gases to dry said first portion of said material to a percentage of moisture content less than said predetermined percentage, separating the exit gases of said first stream from said first portion of material, collecting said separated exit gases of said first stream, supplying a second stream of heated gases, controlling the temperature of said second stream by admitting at least some of said exit gases from said first stream into said second stream in controlled quantities, subjecting a second portion of said material to said second stream of heated gases to dry said second portion of said material to a percentage of moisture content greater than said predetermined percentage, separating the exit gases of said second stream from said second portion or" material, said separated exit gases of said second stream containing less oxygen than said exit gases of said first stream, conducting said separated cxit gases of said second stream through an electrostatic precipitator, and blending said first and second portions of said material to produce a quantity of said material having said predetermined percentage of moisture content.

4. Apparatus for drying a quantity of moist particulate material to a predetermined percentage of moisture content comprising a source of heated gases, a first drying means, first conduit means to conduct a first portion of gases from said source to said first drying means, air inlet means in said first conduit means, first damper means associated with said first conduit means and operable to regulate the flow of air through said air inlet means and thereby control the temperature of gases Within said first drying means, said first drying means being operable to dry a first portion of said material to a percentage of moisture content less than said predetermined percentage, a second drying means, second conduit means to conduct a second portion of gases from said source to said second drying means, third conduit means to conduct the exit gases from said first drying means to said second conduit means, second damper means associated with said second and third conduit means and operable to control the temperature of said gases within said second conduit means by admitting gases from said third conduit means into said second conduit means in controlled quantities, said second drying means operable to dry a second portion of said material to a percentage of moisture content greater than said predetermined percentage, and means to blend the dried first and second portions of said material to produce a quantity of said particulate material having said predetermined percenta e of moisture content.

5. Apparatus for drying a quantity of moist particulate material to a predetermined percentage of moisture content comprising a source of heated gases, at first drying means, first conduit means to conduct a first portion of gases from said source to said first drying means, air inlet means in said first conduit means, first damper means associated with said first conduit means and operable to regulate the flow of air through said air inlet means and thereby control the temperature of gases within said first drying means, said first drying means being operable to dry a first portion of said material to a percentage of moisture content less than said predetermined percentage, a second drying means, second conduit means to conduct a second portion of gases from said source to said second drying means, third conduit means to conduct exit gases from said first drying means to said second conduit means, fourth conduit means to conduct exit gases from said first drying means to said source of heated gases to recycle said exit gases through said source to reheat said exit gases, second damper means associated with said second and third conduit means and operable to control the temperature of said gases within said second conduit means by admitting gases from said third conduit means into said second conduit means in controlled quantities, said second drying means operable to dry a second portion of said material to a percentage of moisture content greater than said predetermined percentage, and means to blend the dried first and second portions of said material to produce a quantity of said particulate material having said predetermined percentage of moisture content.

6. Apparatus for drying a quantity of moist particulate material to a predetermined percentage of moisture content comprising a source of heated gases, first and second gas driers adapted to mix hot gases with particulate material to thereby dry said particulate material, said first and second gas driers each including a drying unit having a, heated gas inlet, a particulate material inlet and an entrained material outlet, said driers each including a separator having an entrained material inlet, a gas outlet and a particulate material outlet, said separators being operable to separate a mixture of gases and entrained particulate material, first conduit means connecting said source of heated gases with said first drying unit heated gas inlet to conduct gases from said source to said drier, first damper means associated with said first conduit means and operable to admit air in controlled quantities into said first conduit means to control the heat of gases Within said first conduit means, first conveyor means to introduce a first portion of said moist particulate material into said first drying unit particulate material inlet, said first drying unit operable to dry said first portion of particulate material to a percentage of moisture content less than said predetermined percentage, second conduit means connecting said first drying unit entrained material outlet and said first separator entrained material inlet to conduct a mixture of gases and entrained particulate material in a stream into said separator, a third conduit means connecting said source of heated gases with said second drying unit heated gas inlet to conduct gases from said source to said drier, fourth conduit means connecting said first separator gas outlet with said third conduit means, fifth conduit means connecting said fourth conduit means with said source of heated gases, second damper means associated with said third and fourth conduit means and operable to admit a first portion of gases from said first separator gas outlet through said fourth conduit means into said third conduit means in controlled quantities to control the heat of gases Within said third conduit means, valve means associated With said fourth and fifth conduit means and operable to admit a second portion of gases from said first separator gas outlet through said fifth conduit means, second conveyor means to introduce a second portion of said moist particulate material into said second drying unit particulate material inlet, said second drying unit operable to dry said second portion of particulate material to a percentage of moisture content greater than said predetermined percentage, sixth conduit means connecting said second drying unit entrained material outlet and said second separator entrained material inlet to conduct a mixture of gases and entrained particulate material in a stream into said separator, an electrostatic precipitator having a dust laden gas inlet and a clean gas outlet, seventh conduit means connecting said second separator gas outlet to said precipitator dust laden gas inlet to conduct gases from said second separator to said precipitator, eighth conduit means connected to said precipitator clean gas outlet to vent clean gases from said precipitator to the atmosphere, a blend hopper having a particulate material inlet and a blended mixture outlet, third conveyor means operable to convey particulate material from said first separator particulate material outlet to said blend hopper inlet, fourth conveyor means operable to convey particulate material from said second separator particulate material outlet to said blend hopper inlet, said blend hopper operable to blend particulate material from said first and second separators into a quantity of particulate material having said predetermined percentage of moisture content. 7. The method of drying a quantity of moist particulate material having a spectrum of sizes to a moisture content of between about 4 and 5 percent comprising the steps of supplying a first stream of gas having a temperature of about 1500 F., tempering said first stream of gas with ambient air to a temperature of about 1000 F., subjecting a first portion of said material to said first stream of heated gas to dry said first portion ofsaid material to a moisture content of about 3 percent, separating the exit gas of said first stream from said first portion of material, supplying a second stream of heated gas at a temperature of about 1500 F., tempering said second stream of heated gas With a portion of said exit gas from said first stream to a temperature of about 1000 F., subjecting a second portion of said material to said second stream of heated gas to dry said second portion of said material to a moisture content of about 8 percent, separating said exit gases of said second stream from said second portion of material, conducting said separated exit gas of said second stream through an electrostatic precipitator, and blending said first and second portions of said material to produce a quantity of said material having a moisture content of between about 4 and 5 percent.

References Cited in the file of this patent UNITED STATES PATENTS 2,023,247 Senseman Dec. 3, 1935 2,132,656 Smith Oct. 11, 1938 2,213,668 Dundas et al Sept. 3, 1940 2,770,052 Morrison Nov. 13, 1956 

1. THE METHOD OF DRYING A QUANTITY OF MOIST PARTICULATE MATERIAL HAVING A SPECTRUM OF SIZES TO A PREDETERMINED PERCENTAGE OF MOISTURE CONTENT, COMPRISING THE STEPS OF SUPPLYING A FIRST STREAM OF HEATED GAS, CONTROLLING THE TEMPERATURE OF SAID FIRST STREAM BY ADMITTING AMBIENT AIR INTO SAID FIRST STREAM IN CONTROLLED QUANTITIES, SUBJECTING A FIRST PORTION OF SAID MATERIAL TO SAID FIRST STREAM OF HEATED GAS TO DRY SAID FIRST PORTON OF SAID MATERIAL TO A PERCENTAGE OF MOISTURE CONTENT LESS THAN SAID PREDETERMINED PERCENTAGE OF MOISTURE CONTENT, COLLECTING THE EXIT GASES OF SAID FIRST STREAM AFTER SAID STREAM HAS DRIED SAID FIRST PORTION OF SAID MATERIAL, SUPPLYING A SECOND STREAM OF HEATED GAS, CONTROLLING THE TEMPERATURE OF SAID SECOND STREAM BY ADMITTING AT LEAST SOME OF SAID EXIT GASES FROM SAID FIRST STREAM INTO SAID SECOND STREAM IN CONTROLLED QUANTITIES, SUBJECTING A SECOND PORTION OF SAID MATERIAL TO SAID SECOND STREAM OF HEATED GAS TO DRY SAID SECOND PORTION OF SAID MATERIAL TO A PERCENTAGE OF MOISTURE CONTENT GREATER THAN SAID PREDETERMINED PERCENTAGE OF MOISTURE CONTENT, SEPARATING SAID SECOND STREAM OF HEATED GAS FROM SAID SECOND PORTION OF SAID MATERIAL, INTRODUCING SAID SECOND STREAM OF GAS INTO AN ELECTROSTATIC PRECIPITATOR TO REMOVE ENTRAINED FINE SIZED DUST-LIKE MATERIAL THEREFROM, AND BLENDING SAID FIRST AND SECOND PORTIONS OF SAID MATERIAL TO PRODUCE A QUANTITY OF SAID MATERIAL HAVING SAID PREDETERMINED PERCENTAGE OF MOISTURE CONTENT. 